You are probably familiar with magnetic resonance imaging machines, either through the claustrophobic
sensation of being inside one or from seeing the incredibly high detail cross sections
of the human body they provide.
.
MRIs have completely changed how we view and understand our bodies.
It has never been easier to visualize organs with such high details.
We can safely locate and identify tumors in the kidneys, brain, stomach, and pancreas.
We can inject paramagnetic contrast agents into the bloodstream to locate blockages in
the heart, allowing doctors to accurately implant life-saving stents to open blood vessels.
A tiny keyhole surgery that patients can recover from quickly, where once dangerous open heart
surgery was the only option.
Magnetic resonance imaging has truly changed the nature of medical diagnosis and treatment.
A technology that seems straight out of a sci-fi novel, how they work is a complete
mystery to most.
To unlock the power of this machine physicists and engineers first had to discover and master
the principles of quantum mechanics, superconducting magnets, computer science, and mathematics.
The clean futuristic white facade of an MRI machine hides a world of complicated and marvelous
engineering that you may not have ever considered.
Prior to their introduction to medicine, we peered into our bodies using harmful ionizing
x-rays or low-detailed ultrasounds.
While incredibly useful, even to this day, these two imaging techniques pale in performance
to the safe, millimeter resolution of MRIs.
Capable of creating a 3D reconstruction of the body rather than a flat 2D image.
Achieving all of that without any moving parts.
Imaging in medicine relies on collecting signals from the body based on the innate physical
properties of tissues.
Ultrasound imaging relies on how sound waves bounce off tissues of different densities.
X-rays form images based on the absorption of high-energy radiofrequency waves.
Magnetic resonance imaging, however, relies on something far less intuitive, the quantum
properties of the hydrogen atom.
The human body is teeming with hydrogen atoms
located in water, carbohydrates, and proteins.
To image the body, MRI machines exploit a quantum property of these hydrogen atoms.
A property called spin.
How it does this is a marvel of modern physics and engineering.
Spin is an innate property of particles just like mass and charge.
Spin makes particles behave like tiny bar magnets.
The proton inside the hydrogen nucleus behaves like a magnet.
The orientation of its magnetic north is described probabilistically.
Under normal circumstances, this probability is evenly distributed.
This means that the combined magnetic field of many hydrogen atoms cancels out.
But, this changes when the hydrogens are inserted into a large external magnetic field just
like the ones MRIs create.
This changes the distribution of the tiny magnets, augmenting the number of atoms aligned
with the external magnetic field.
This imbalance is the source of the MRI signal, as we can manipulate these tiny magnets to
produce signals that can be processed into images.[1]
Once the atoms are aligned in the machine's incredibly strong magnetic field the machines
give them a tiny nudge using a magnetic radiofrequency pulse.
This pulse comes from this set of coils inside the machine that send pulses by simply applying
an alternating current through its coils at a very specific frequency.
This nudge misaligns these hydrogen bar magnets to shift their magnetic field perpendicular
to the large magnetic field the machine is creating.
Naturally, the spins want to align their orientation back to their original position, aligned with
the machine's magnetic field, but they don’t fall back immeditely, they decay in a spiraling
motion.
This decay causes a changing magnetic field, and by placing a coil of wire nearby, this
changing magnetic field can induce currents that can be read as a clear signal.
MRI machines can use the same coils to send the “nudge pulses” and to read the signal
from the body.
These coils are placed here, as close to the patient as possible but still Inside the MRI
tube.
For higher contrast and resolution, some machines use separate coils to transmit pulses and
receive the signal.
This allows the receiver coils like these to be placed much closer to the body, maximizing
the strength of the signal.
[2]
We can increase the number of hydrogens, aligning with the external magnetic field by increasing
the strength of the external magnetic field.
In turn, the hydrogen magnet can induce a larger current as it spirals back.
So, by simply increasing the strength of the MRIs field, we increase the strength of the
signal collected and therefore improve image quality.
Common MRI field strengths are 1.5 to 3 tesla, around 300,000 times stronger than the earth's
magnetic field and 30,000 times stronger than your common fridge magnet. [3] You absolutely
do not want anything ferrous around these machines during operation, a field this strong
can lift nearby wheelchairs straight off the ground.
For research purposes, MRIS can produce even higher magnetic fields, up to 20 teslas.
Achieving this intense magnetic field comes as no easy feat.
Early MRIs used permanent magnets as their source of the main magnetic field but only
reached strengths of 0.5T limiting the resolution of the machine [4].
Electromagnets can be used to reach stronger magnetic fields, but standard electromagnets
can’t produce a 1.5 tesla field.
Higher magnetic fields require higher electric currents that would melt ordinary wires.
To achieve larger currents in the wires, engineers required superconducting coils. [4]
Superconductors are another sci-fi technology.
Temperature affects all metallic conductors.
With resistance gradually lowering with temperature.
But superconducting materials are special, in that their resistance drops to zero at
temperatures close to minus 273 degrees Celsius, or absolute zero.
In theory, when this happens, an electric current could travel in a loop of superconducting
material indefinitely, never needing a power source.
In reality, this means that the main superconducting coil in MRIs does not consume any power directly.
Rather, the main consumption of energy is just to keep the coil cooled down so the current
will travel endlessly, leaving the MRI magnet permanently on.
The energy needed to run an MRI for a full year is equivalent to 25 four-person households,
around 130,000 to 140,000 kWh per year.
[5]
The most common superconducting material used in MRIs is Niobium–titanium.
[6].
The demand for High-resolution images is so large that 80% of all the Nb-Ti we extract
from the earth goes into an MRI machine [7].
To achieve the incredibly low temperatures needed for superconductivity, we need a very
cold refrigerant.
Early MRI machines used to submerge their superconducting wires in a bath of liquid
Helium.
Pouring one thousand (1000) liters of liquid helium, at minus 269 degrees celsius, into
the machine to cool the superconducting coil as close to absolute zero as possible.
This evaporated the helium, allowing it to escape the machine as a gas.
Meaning, early MRI machines required regular refillings of liquid helium.[8]
Even though helium is an incredibly common gas in the universe, it is so light that it
can escape our atmosphere into space.
We extract helium from underground gas caverns, where it accumulated as a byproduct of uranium
and thorium radioactive decay.
But once we allow that helium to escape into the atmosphere, it is gone for good.
Floating to the top of our atmosphere and gradually being blown into space by solar
winds.
We will eventually run out of natural helium.
This method of cooling was costly and unsustainable, costing up to $26,000 per year in helium refills.[9]
To avoid this refilling problem, modern MRI machines use a vacuum-sealed chamber that
holds the liquid helium without letting it evaporate.
This eliminates the need for refilling and minimizes the cost of operation.
These so-called “Zero Boil off” machines are now the norm in MRI technology.
They use an electric refrigerant cycle like the one in your fridge but on steroids.
This cycle keeps the helium in its liquid phase and keeps the magnets cool enough to
maintain them in their superconducting state [8].
Now that we know why and how the hydrogen atoms are aligned, we need a way to turn that
information into an image, and to do that we need to know the physical location of the
hydrogen atoms.
Recall that MRIs detect signals from the spiral decay pattern of hydrogen atoms after they
have been “nudged”.
These spirals decay with a unique rotational frequency (⍵),. interestingly, this is the
only frequency that can “nudge” the hydrogen atoms too.
Luckily, rotational frequency is dependent on something we control, the magnetic field
strength.
For example, hydrogen resonates at 64 MHz at 1.5 Tesla and at 128 MHz at 3 tesla[10].This
means the atoms in a weaker magnetic field will rotate slower while atoms in a stronger
field will rotate faster.
To nudge these atoms we need a radiofrequency wave at 64 MHz or 128 MHz.
We can use this to our advantage to image individual slices.
If we can apply a gradient to the magnetic field strength, we can selectively nudge atoms
along the gradient by applying the corresponding frequency.
This gradient is applied precisely using a separate set of regular electromagnets, aptly
named gradient coils.
To image a slice near the weaker end of the tube, the machine sends a “nudging pulse”
centered at a lower rotational frequency say 63.998 MHz and to image a slice on the other
side of the tube the pulse is centered at 64.002MHz.
This still feels like magic.
It’s not terribly obvious how detecting the decay from these spiraling hydrogen atoms
can create images.
The receiving coil can only measure the sum of all these spiraling decays.
We need a way of processing the signal to create an image.
Let's start by understanding how to get contrast between tissues before delving deeper into
the techniques used to actually form images.
We need a way of identifying different kinds of tissues to form an image, to do that we
need to contrast the tissues using two different signal types.
The first one deals with how quickly atoms re-align themselves with the large magnetic
field after a nudging pulse.
This is called T1 relaxation.
The second measure comes from the physical reality of interaction between hydrogens.
The atoms do not realign with the magnetic field uniformly.
In tissue, hydrogens interact with each other and with their surroundings.
Right after the nudging pulse is sent, the small interactions cause the spins to fall
out of uniformity.
Since the coil can only measure the sum of all these spiral decays, then the failing
out of order would create a decaying signal.
This is called T2 decay.
Importantly, the two rates are not equal, and even more important they are dependent
on the tissue.
Hydrogens in fat have different intrinsic characteristics and interactions than hydrogens
in water.
This difference is what lets technicians contrast the tissues.[10]
We can emphasize the T1 signal by sending pulses rapidly and listening to the signal
immediately, as the dephasing effects of T2 do not have enough time to take place.
We can emphasize T2 by sending pulses slowly and listening for longer, allowing the dephasing
to occur.
[11]
Just like a photographer can play with camera settings to take pictures of either the bright
sky or a dim environment.
MRI technicians can also play with two settings to take images of contrasting tissues, the
time between pulse repetition, and how long to wait to listen for a signal.
These two parameters are chosen by the technician each time an MRI is performed and they choose
them depending on what the doctor would like to image.[10]
For example, T1 is used to image fatty tissues while suppressing the signal from water.
But maybe the doctor wants to assess the cerebral spinal fluid in the spine or the brain, and
for that T2 signals are emphasized to enhance the signal from water-based fluids.[12]
Remember, the signal is collected as a sum from a singular slice, The machine needs to
form a 2D image of each slice.
There is one extra layer of complexity since the receiver coil can only measure the sum
of the signals from all the decaying hydrogens in the slice.
This is yet another story where old mathematics discoveries guide modern technology.
In 1822 Joseph Fourier created a mathematical framework that deconstructs complex waves
into simple additions of simpler waves.
This framework can be extended to 2D grayscale geometries.
Just like a musical melody can be simplified into additions of simpler notes.
Any image can be deconstructed into a weighted average of simpler black and white stripes.
This is what MRIS uses to create images.
Instead of sampling individual pixels, MRIs sample different striped patterns.
In reality, what does this mean for the spinning hydrogens?
What is the physical representation of these patterns?
It is easier if we imagine the slice as a grid of rotating hydrogens where we color
the phase of the atoms in grey scale.
When rotating in unison the grid is all white.
Atoms rotating 180 degrees out of phase are colored black.
MRIs exploit this to physically create the striped patterns needed for the Fourier analysis.
Using another set of gradient coils MRIs precisely change the phase of rotating hydrogens to
create striped patterns.
By controlling how long the Y and the X gradients are turned on, the MRI can create patterns
in all directions and frequencies.
Slowly the machines start to imprint all the patterns with different frequencies and orientations
to sample their relative strengths in physical space.
And slowly by adding more and more patterns the image starts to emerge.
This is what forms the image for each 2D slice.
Then the machine goes on to the next slice of the body and starts the process all over
again.
Turning on and off coils inside a magnetic field might sound familiar if you’ve ever
played with a speaker.
As basically they are the same things.
Gradient coils rapidly turning on and off are the source of the loud clunking sounds
in MRIs.
MRIs are constantly evolving and the core technology is starting to branch off.
Researchers want the highest resolution and strive to increase the strength of the magnet,
while some hospitals and companies are making them smaller and cheaper, as more practical
and cheap MRIS are incredibly useful in the field.
[13]
It's hard to fathom the complexity of all these systems, from superconducting wires
to vacuum-sealed helium reserves to rapidly changing magnetic gradients, and even more
incredible to think of the first people who figured out how to combine these technologies
together to peer into our bodies.
This intricate dance of quantum physics and carefully manipulated gradients has allowed
MRIs to change the world of medicine.
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